Three-dimensional Density Field Research Articles

Supersonic Turbulence in the Interstellar Medium: Stellar Extinction Determinations as Probes of the Structure and Dynamics of Dark Clouds

Lada et al. have described a method for studying the distribution of dust in dark clouds using infrared imaging surveys. In particular, they show that the method provides some information about the structure of the gas (dust) on scales smaller than their resolution. In the present work we clarify the nature of the information provided by their method. We show that: 1. The three-dimensional density field of the gas is well described by a lognormal distribution down to very small scales. 2. The power spectrum and the standard deviation of the three-dimensional density field can be constrained. 3. Such a structure of the density field is likely to be the effect of random supersonic motions present in the gas. In fact, we find a qualitative and quantitative agreement between the predictions based on recent numerical simulations of randomly forced supersonic flows by Nordlund & Padoan and by Padoan, Nordlund, & Jones and the constraints given by the infrared dust extinction measurements.

The Study of Transonic Flows by Quantitative Holographic Interferometry

Optical flow diagnostics are non-intrusive and therefore suitable for investigating perturbation sensitive transonic flows. Unfortunately, most of these techniques depend on particle seeding and are confined to point measurements. In contrast, holographic interferometry is whole field, does not require seeding, and is an appropriate tool for the study of temporal and spatial properties of unsteady flows. At present, however, holographic interferometry is mainly used qualitatively due to the practical restrictions of data recording, acquisition and processing. This paper addresses some of the limitations that have prevented routine use of holographic interferometry in production wind tunnel testing. Vibration is a frequent problem when capturing interferometric data; however, this paper shows that the associated rigid body movements are easily removed during Fourier transform fringe analysis. A second major limitation arises when attempting to interpret and analyse interferograms from three-dimensional flows. To overcome this problem, a prototype optical system is described which can record multiple interferometric views around the working section. The information extracted from these images may then be used to tomographically reconstruct the three-dimensional density field. Holographic measurements for two-dimensional and three-dimensional axisymmetric flows are presented and compared quantitatively to computational fluid dynamics (CFD) predictions. The experimental and computational results are in general agreement and demonstrate the feasibility of automating the extraction of three-dimensional density data from holographic interferograms.

A numerical experiment on computations of the Black Sea density fields and current velocities during summer

A near-bottom pressure equation consistent with the original set of difference equations is derived in the framework of a numerical energetically-balanced model. The model is used to simulate numerically three-dimensional density and current velocity fields and in the Black Sea for a specific season. The climatic values of temperature, salinity, and tangential wind stress during summer are applied as the original conditions. Analysis of the hydrophysical fields at the final moment of integrating is performed. The difference between new data and data derived earlier is demonstrated.

Potential, velocity, and density fields from sparse and noisy redshift-distance samples - Method

A method for recovering the three-dimensional potential, velocity, and density fields from large-scale redshift-distance samples is described. Galaxies are taken as tracers of the velocity field, not of the mass. The density field and the initial conditions are calculated using an iterative procedure that applies the no-vorticity assumption at an initial time and uses the Zel'dovich approximation to relate initial and final positions of particles on a grid. The method is tested using a cosmological N-body simulation 'observed' at the positions of real galaxies in a redshift-distance sample, taking into account their distance measurement errors. Malmquist bias and other systematic and statistical errors are extensively explored using both analytical techniques and Monte Carlo simulations.

Simulating the Main Thermocline in the North Atlantic with an Ideal-Fluid Model

Abstract Using an ideal-fluid model, the main thermocline in the North Atlantic is reconstructed. The new feature in the model is the use of a nonlinear background stratification calculated from the hydrographic measurements collected on the Discovery cruise along 46–49°N. The basic stratification is assumed to be set up by an external thermohaline circulation and modified by a wind-driven circulation superimposed upon it. The numerical results of the model distinctly show the three-dimensional structure of the main thermocline in the ocean. In addition, the wind-driven gyre reaches a great depth (≈4.5 km), thus topographic effects on the gyre-scale circulation must be considered in further models. The model confirms the classical notion that the basic structure of the main thermocline and the three-dimensional density and velocity fields in the upper ocean can be simulated very well without explicitly including friction in the model.

Measurement of a three-dimensional temperature field applying ESPI and CT techniques

Electronic speckle pattern interferometry (ESPI) and computer tomography (CT) techniques are combined to measure a three-dimensional temperature or density field. The method is especially applicable to measurements on asymmetrical fields. Theory and experimental results are presented.

A Three-Dimensional Simulation of the Hudson–Raritan Estuary. Part II: Comparison with Observation

Abstract Results from a time-dependent, three-dimensional numerical simulation of the Hudson–Raritan estuary are compared with observations. The comparison includes: 1) instantaneous salinity contours across a transect in the estuary; 2) amplitudes and phase of tidal constituents at four tide gauge and five current meter stations, 3) mean currents at nine meter locations, and mean salinity in the Hudson River, 4) kinetic energy spectra; and 5) response to wind forcing of subtidal current at an observational station near the mouth of the estuary. Observations confirm the model's prediction of existence of density advection instabilities induced by differential advection of the three-dimensional density field. These instabilities produce intense vertical mixing and should significantly modify dispersion processes in the estuary. Effects of neap–spring tides on vertical stratifications are also simulated by the model. Simulated M2 phase at three tide gauge stations show improvement over the M2 phases obtaine...

Quantitative Determination of Three-Dimensional Density Field by Holographic Interferometry

principal objective of this paper is to report a successful application of holographic interferometry to a large-scale wind-tunnel experiment. Emphasis is placed on the quantitative determination of the threedimensional density field under investigation. A rational procedure is developed which makes the proper use of an efficient data-reduction theory to calculate three-dimensional density fields around opaque bodies. This new procedure eliminates the major difficulty in three-dimensional datareduction involving opaque objects, and is used to reduce the three-dimensional density data from the interferograms produced in the experiment. Good agreement between experiment and theory is achieved.

Application of Holographic Interferometry to Density Field Determination in Transonic Corner Flow

The present report is believed to be the first quantitative measurements obtained by holographic interferometry of the density field in transonic corner flow. The technique offers a new method to obtain quantitative transonic flow measurements without disrupting the flow. Finite fringe interferometry has been used to investigate the threedimensional density field about a partially transparent wing-body structure. The resulting asymmetric density field and shock wave structure are shown to be an accurate representation of the phenomena encountered in aerodynamic corner flow. The double pulse technique allows the interferogram to be taken through the transparent nonflat optical elements in the wing. The depth focusing and angular viewing properties of a hologram although of possible use in transonic flows were not used in the present experiment.

Finite Fringe Holographic Interferometry Applied to a Right Circular Cone at Angle of Attack

The successful application of holography to the study of three-dimensional flow fields due to phase objects has been reported in the literature. The present report extends this technique to the study of density fields around opaque bodies as would normally be encountered in wind tunnel experiments. The density field around a 10-deg half-angle cone at 0 and 10-deg angle of attack has been investigated by means of the finite fringe holographic interferometry. The three-dimensional density field obtained from the reduction of the interferograms was found to agree with that obtained from an analytical solution of the governing equations.

Determination of Three-Dimensional Density Fields from Holographic Interferograms

The successful application of holographic interferometry, and an associated mathematical reduction process, to the determination of an asymmetric three-dimensional density field of an aerodynamic phenomenon is reported. An integral inversion method from the field of plasma physics has been extensively evaluated by applying it to the determination of functions, both axisymmetric and asymmetric, which simulate aerodynamic density fields. The application of holographic interferometry has been extended to provide multiple holograms about a test region, with sufficient coverage to provide interferometric data for the successful solution of the density field. The analytical and experimental methods developed were applied to an experimental axisymmetric test field, the supersonic flow from a free jet, and shown to be comparable to a previous solution obtained by the Abel inversion method. Further, the free jet was tilted to provide a test field which was asymmetric in the plane of solution. Comparison of the resulting asymmetric solution was shown to be consistent with the previously obtained axisymmetric solution.

A Nonlinear Model of an Ocean Driven by Wind and Differential Heating: Part I. Description of the Three-Dimensional Velocity and Density Fields

A numerical experiment is carried out to investigate the circulation of an ocean, driven by a prescribed density gradient and wind stress at the surface. The mathematical formulation includes in one model most of the physical effects that have been considered in previous theoretical studies. Starting out from conditions of uniform stratification and complete rest, an extensive numerical integration is carried out with respect to time. Care is taken in the final stages of the calculation to use a finite difference net which resolves the very narrow boundary layers which form along the side walls of the basin. A detailed description is made of the three-dimensional velocity and temperature patterns obtained from the final stage of the run. Since inertial effects play an important role in the western boundary current, it is possible to verify with a baroclinic model two results obtained previously with barotropic ocean models: 1) a concentrated outflow from the western boundary takes place along the upper boundary of the subtropic wind gyre; and 2) inertial recirculation may increase the total transport of the boundary current to a value well above that given by linear theory. In addition to the western boundary current, a strong eastward flowing current is found along the equator. Taking into account a difference in Rossby number between model and prototype, the intensity of the computed currents agrees very closely to observations in the Gulf Stream and the Equatorial Current.

Photoelectric Photometry of the e0 Galaxy NGC 3379.

view Abstract Citations (87) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric Photometry of the e0 Galaxy NGC 3379. Miller, R. H. ; Prendergast, K. H. Abstract Detailed photometric maps of the E0 galaxy NGC 3379 were made by means of photoelectric techniques. Approximately 400 points were measured in each of two colors. The galaxy is of quite uniform color except for a small region within about 20 seconds of arc of the center, which is a little redder- A(B - V) 0 06. The three-dimensional volume emissivity was calculated from the photometric results by a X2-minimum process. Two independent methods are proposed for calculating the three-dimensional mass density and gravitational potential fields from the volume emissivity. The gravitational potential fields calculated according to each of these methods are compared for the case in which the symmetry axis is assumed to be perpendicular to the line of sight with satisfactory agreement out to a radius within which half of the mass is presumed to reside. The departure beyond this point is discussed. A very preliminary discussion of the implications of this work concerning the structure of the galaxy is given. Under certain assumptions which may be of doubtful validity, we find that the mass of this galaxy is about 1011 solar masses and the mass-to-light ratio is about 9. Publication: The Astrophysical Journal Pub Date: November 1962 DOI: 10.1086/147431 Bibcode: 1962ApJ...136..713M full text sources ADS | data products NED (1)

Analysis of Turbulent Density Fluctuations by the Shadow Method

Shadow pictures of turbulent flow regions give a particular kind of information about the three-dimensional density field. If homogeneity and isotropy of the fluctuation field can be assumed, the statistical properties of the picture uniquely determine the correlation function or power spectrum of the three dimensional turbulent density field. Shadow pictures obtained in the wake of a supersonic projectile were analyzed in this manner and encouraging results were obtained. In order to check the validity of the method, the density fluctuations in a heated jet were measured both by the shadow method and by the more conventional hot-wire technique.